Transmission media: Twisted pair, Coaxial, Fiber optic, Line-of-site, Satellite

Twisted pair cables

Twisted pair cables are constructed of multiple pairs of twisted cables contained by common jackets. When wires are twisted together, twists reduce the cable’s sensitivity to outside EMI.

Twisted pair is of two types:-

Unshielded twisted pair

UTP contains no shielding and is more susceptible to external noise but is the most frequently used because it is inexpensive and easier to install.

Shielded Twisted Pair

STP cable contains an outer conductive shield that is electrically grounded to insulate the signals from external electrical noise. STP also uses inner foil shields to protect each wire pair from noise generated by the other pairs.

Coaxial cable

Coaxial cable is a type of shielded cable. It consists of a solid copper conductor surrounded by insulating material and a braided conductive shield. It has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield.

Fiber-optic

Fiber-optic cable uses light to transfer data over a physical medium. Fiber-optic cable increases and decreases the intensity of light to represent binary ones and zeros in data transmissions. The strength of a light signal does not diminish as much as the strength of an electrical signal does over an identical run length. Fiber-optic is immune to EMI. Bandwidth is in the range of the Gbps. For long distance single mode Fiber optic cable is used which uses LASER as a light source and for shorter distance Multi Mode is used which used LED as light source.

Line-of-site Propagation

Line-of-sight propagation refers to electromagnetic radiation or acoustic wave propagation. The rays or waves travel in a straight line which is diffracted, refracted, reflected, or absorbed by atmosphere and obstructions with material and generally cannot travel over the horizon or behind obstacles. The two communicating antenna must be placed such a way that they see each other in earth's curvature.

Satellites

Satellites are placed in earth orbit for the purpose of communicating, weather forecast, research, military purpose, etc. Satellites consist of transponders that gather signals over a range of uplink frequencies and re-transmits them on a different set of downlink frequencies to receivers on Earth, often without changing the content of the received signal. Satellites are classified based on the altitude of orbit as:

• LEO (Low Earth Orbit) Satellites: These satellites are kept below 5000 km. As the satellites are so close to the earth, the ground stations do not need much power, and the round-trip delay is only a few milliseconds. But due to their rapid motion, large numbers of them are needed for a complete system.
• MEO (Medium-Earth Orbit) Satellites: These satellites are kept in the range of 5000 to 15000km. They must be tracked as they move through the sky. Because they are lower than the GEOs, they have a smaller footprint on the ground and require less powerful transmitters to reach them.
• GEO (Geostationary) Satellites: GEO satellites have rotation period of 24hrs so they are stationary with respect to Earth. They have kept at altitude approx. 36000Km above equatorial plane.

References:

1. A.S. Tanenbaum, “Computer Networks”, 3rd Edition, Prentice Hall India, 1997.
2. W. Stallings, “Data and Computer Communication”, Macmillan Press, 1989.
3. Kurose Ross, “Computer Networking: A top-down approach”, 2nd Edition, Pearson Education
4. Larry L. Peterson, Bruce S. Davie, “Computer Networks: A Systems Approach”, 3rd Edition, Morgan Kaufmann Publishers